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Velocity profiles of a line of H$_3^+$ at 3.7 $\mu$m produced in warm diffusegas have been observed toward 18 stars in the Central Molecular Zone (CMZ) ofthe Galaxy. Their longitude-velocity diagram indicates that the gas is radiallyexpanding within the CMZ at speeds up to a maximum of $\sim$150 km s$^{-1}$.The current momentum and energy in the gas are $\sim 5 \times 10^8 M_\odot$ kms$^{-1}$ and $\sim 5\times 10^{53}$ erg. The motion is similar to that of theExpanding Molecular Ring (EMR) discovered in 1972 by Kaifu et al. and byScoville. We propose that the expanding gas seen in H$_3^+$ is part of the samephenomenon, in spite of differences in estimates of density, morphology, anddegree of rotation. The outward motion suggests that one or more ejectionevents occurred near the center of the CMZ (0.5$-$1) $\times$ 10$^6$ years ago,which may be related to creation of the recently observed microwave bubble.These observations revive the circular face-on view of the CMZ proposed in1972, which fell out of favor after 1991 when Binney et al. proposed that aface-on view of the CMZ would show it to have an elliptical shape, with higheccentricity. While that model may apply on kiloparsec scales, we argue that itis incorrect to apply it to the much smaller CMZ. We discuss the fate of theexpanding gas, which appears to be eventual infall into the center, leading toepisodes of star formation and violent events associated with accretion ontoSgr~A$^\ast$.

The Central 300 pc of the Galaxy probed by infrared spectra of H3+ and CO: II. Expansion and morphology of the warm diffuse gas